A trove of new images captured in the past few days show what is likely to be melted nuclear fuel from inside one of Japan’s wrecked Fukushima reactors, a potential milestone in the cleanup of one of the worst atomic disasters in history.

Tokyo Electric Power Co. Holdings Inc., Japan’s biggest utility, released images on Saturday of mounds of black rock and sand-like substances at the bottom of the No. 3 reactor containment vessel at Fukushima, which is likely to contain melted fuel, according to Takahiro Kimoto, an official at the company. A survey on Friday found black icicles hanging from the above pressure vessel, which was “highly likely” to contain melted fuel. Kimoto noted it would take time to confirm whether this debris contains melted fuel.

“The pictures that we have gained will assist us in devising a plan for removing the melted fuel,” Kimoto told reporters Saturday night in Tokyo. “Taking pictures of how debris scattered inside of the reactor was a big accomplishment.”

If confirmed, these pictures would be the first discovery of the fuel that melted during the triple reactor accident at Fukushima six years ago. For Tokyo Electric, which bears most of the cleanup costs, the discovery would help the utility design a way to remove the highly-radioactive material.

The pictures were taken by a Toshiba-designed robot the company sent to explore the inside of the reactor for the first time from July 19. The robot, 30 centimeters (12 inches) long that can swim in the flooded unit, was tasked with surveying the damage inside and also finding the location of corium, which is a mixture of the atomic fuel rods and other structural materials that forms after a meltdown.

“It is important to know the exact locations and the physical, chemical, radiological forms of the corium to develop the necessary engineering defueling plans for the safe removal of the radioactive materials,” said Lake Barrett, a former official at the U.S. Nuclear Regulatory Commission who was involved with the cleanup at the Three Mile Island nuclear power plant in the U.S. “The recent investigation results are significant early signs of progress on the long road ahead.”

Because of the high radioactivity levels inside the reactor, only specially designed robots can probe the unit. And the unprecedented nature of the Fukushima disaster means that Tepco, as the utility is known, is pinning its efforts on technology not yet invented to get the melted fuel out of the reactors.

The company aims to decide on the procedure to remove the melted fuel from each unit as soon as this summer. And it will confirm the procedure for the first reactor during the fiscal year ending March 2019, with fuel removal slated to begin in 2021.

This underwater photo shows a part of a control rod drive of Unit 3 at Fukushima Dai-ichi nuclear plant in Okuma town, northeastern Japan Wednesday, July 19, 2017.International Research Institute for Nuclear Decommissioning via AP

Decommissioning the reactors will cost 8 trillion yen ($90 billion Canadian dollars), according to an estimate in December from the Ministry of Economy, Trade and Industry. Removing the fuel is one of the most important steps in a cleanup that may take as long as 40 years.

Similar to the latest findings in the No. 3 reactor, Tepco took photographs in January of what appeared to be black residue covering a grate under the Fukushima Dai-Ichi No. 2 reactor, which was speculated to have been melted fuel. However, a follow-up survey by another Toshiba-designed robot in February failed to confirm the location of any melted fuel in the reactor after it got stuck in debris.

A robot designed by Hitachi-GE Nuclear Energy Ltd. also failed to find any melted fuel during its probe of the No. 1 reactor in March.

The significance of the recent finding “might be evidence that the robots used by Tepco can now deal with the higher radiation levels, at least for periods of time that allow them to search parts of the reactor that are more likely to contain fuel debris,” M.V. Ramana, professor at the Liu Institute for Global Issues at the University of British Columbia, said by email.

“If some of these fragments can be brought out of the reactor and studied, it would allow nuclear engineers and scientists to better model what happened during the accident.”